Method of producing high gloss mineral-coated



H. FROST El AL ING HIGH GLOSS MINERAL-COATED AND RESULTANT PRODUCT 1Filed March 20, 1953 Sept. 19, 1961 METHOD OF PRODUC PAPER Origins.

Q E2 z- 0503 0. 5 30m w mwuE m cm 4 -01 mun w W :2: 2.28 mix-mi m 3United States Patent F 25,039 METHOD OF PRODUCING HIGH GLOSSMINERAL-COATED PAPER AND RESULT- ANT PRODUCT Frederick H. Frost,Portland, and Philip S. Leighton,

Westbrook, Maine, assignors to S. D. Warren Company, Boston, Mass, acorporation of Massachusetts Original No. 2,759,847, dated. Aug. 21,1956, Ser. No.

343,584, Mar. 20, 1953. Application for reissue May 19, 1958, Ser. No.736,422

4Claims. (Cl. '11764) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention relates to the production of mineralcoated sheet materialcoated on one or both sides and having a very high gloss and a verylevel surface, and to the glossy mineral-coated sheet material soproduced.

In the production of mineral coated paper a paper base which has beensurface-sized, surface-filled, or which may be coated or uncoated, iscoated with an aqueous composition comprising mineral pigment such asclay, blanc fixe, calcium carbonate, talc, titanium dioxide, variouscolored pigments or the like and adhesive material such as casein,starch, glue, poylvinyl alcohol, synthetic elastomer, or the like. Thecoated surface is then dried and smoothed. Customarily from a minimum ofabout one pound to a maximum of about seven pounds of dry weight ofcoating is applied to each 1000 sq. ft. of surface coated. The lighterweights of coating are used if the base paper has been previously coatedwhereas the heavier weights are used on base paper which has not beenpreviously coated or surface filled. Thus the coating may be applied inone layer or in two or more layers as desired; either one or both sidesof the web may be coated. Most coated papers are first dried and thensmoothed by calendering and such papers constitute the standard glossycoated papers of commerce. Such papers possess an attractive sheen butdo not have a high specular or mirror-like gloss approximating, forinstance, that of polished metal surfaces.

For producing a high specular gloss on coated paper surfaces which arenormally used for commercial printing purposes there are presently inuse only three commercial methods, namely: flint-glazing, buffing asdisclosed by Clark Patent No. 2,349,704, and cast coating as disclosedby Bradner Patent No. 1,719,166 and Warner Patent No. 2,316,202. Afourth method of producing glossy-coated paper is by friction-glazing,by which the coated surface is burnished by a rapidly turning metalroll, usually heated. The result so produced is somewhat similar, butgenerally quite inferior, to that obtained by flint-glazing. Generallyspeaking, the friction-glazed product is not sufficiently high inspecular gloss to be in the same class as the products previouslymentioned.

The present invention has for its object the production ofmineral-coated paper having high specular gloss at least comparable tothat of paper produced by the before mentioned known methods.

The present invention is based on the discovery that the application ofaqueous liquid to the surface of a hard, dry, densified supercalenderedcoating comprising hydrophilic material on a sheet of paper just as itpasses into the nip between a highly polished roll and a backing rollwith the coating facing the polished roll will cause the hydrophilicmaterial at the surface of the coating to swell, become moldable and byexpansion to be pressed against and molded by the polished roll, therebyreceiving a surface which is complementary to that of the polished rolland that the novel paper so produced may have a gloss Reissued Sept. 19,1961 at least equal to that made by any other process of which we areaware.

According to the invention a paper web or other flexible sheet materialis coated by any desired means, e.g., roll coater, brush coater, etc.,with one or more applications of an aqueous hydrophilic coatingcomposition comprising a finely divided coating pigment such as clay, ahydrophilic material and sufficient adhesive material to bind thepigment to the paper base. The hydrophilic material and the adhesivematerial may be identical or different. Suitable hydrophilic materialsinclude casein, starch, soy protein, glue and polyvinyl alcohol, whichmay be used singly, mixed with each other, or mixed with a substantialproportion of an elastomer such as synthetic rubber latex. The paper webso coated is then dried and densified by calendering, under considerablepressure, as by a supercalender. It is not necessary to calender enoughto develop high gloss, but it is necessary to use enough pressure todensify the coating and it is advantageous to render the surface fairlylevel; it is permissible, but not essential, to calender until a highsupercalendered gloss is obtained. The weight of coating of the finishedpaper may be the same as in the prior methods referred to.

In contrast to this procedure in the method of cast coating described inPatents 1,719,166 and 2,316,202, the cast coated paper is neitherair-dried nor calendered. Instead, the wet freshly coated paper isrolled tightly against the surface of a heated drum and is there dried.Thus cast-surfaced coated paper is made without the use of asupercalender with its high pressures and the coating on its surface hasa higher bulk and greater ink absorbency than paper which is madeaccording to the present invention.

In the method of this invention, the densified coated surface is wetwith an aqueous liquid as the paper passes into the nip of a pair ofrolls one of which is heated and has a polished surface complementary tothat of the glossy surface of the paper to be produced. The other rollis preferably a rubber-covered backing roll. The aqueous liquid isapplied to the densified surface as closely as possible to the nip ofthe two rolls, so that the swelling of the hydrophilic component of thecoating takes place as nearly coincidentally as possible with thepassage of the wetted portion of the coating into and through the nip ofthe rolls, and before the aqueous liquid has had time to penetrate thecoating and pass into the backing sheet. The coating remains in contactwith the heated polished surface until it has dried. When dry, the papercomes away easily from the polished support and has a surface which issubstantially the mirror image of the polished roll surface. If the rollsurface is highly polished, the coated paper dried against it willexhibit an extremely high gloss.

The most satisfactory way of applying an aqueous liquid to the coatingwithout penetration to the backing sheet is by keeping the nip betweenthe paper and the polished roll surface filled or flooded with aqueousliquid. Liquid may be injected directly into the nip, or it may beapplied to the polished roll surface before the latter comes intocontact with the surface of the coating or both in quantity sufficientto keep the nip filled and flooded.

With some coatings water alone is an entirely satisfactory wettingliquid; with other coatings which are more water-resistant, such ascoatings containing a considerable proportion of a synthetic elastomeror partially waterproofed casein, it is advantageous to use an aqueousso-' tity of an anti-sticking or parting agent to facilitate release ofthe dry coated surface from the polished surface a'fter it has dried incontact therewith. Soaps and solid polyethylene glycol are usefulanti-sticking agents. If the dried coating parts from the polishedsurface with difficulty, parting may be facilitated by adding a smallquantity of anti-sticking agent to the aqueous liquid in the: nip; atrace of glycol monolaurate or ammonium stearate dissolved in theaqueous liquid is found very effective.

I The best results appear to. be obtained when the following conditionsare observed:

(.1.)That, the sheet material used bear on at least one side, a surfacelayer of mineral-coating containing some hydrophilic material,preferably a hydrophilic adhesive material, which is swellable byabsorption of aqueous liquid.

(2)v That the coating be densified and the gross irregularities inthelevelness thereof removed.

(3). That the coated sheet be dry, e.g. substantially airdry, beforeaqueous liquid is applied to the surface thereof according to theinvention.

(4) That the application of aqueous liquid to the surface of the coatingtake place as nearly simultaneously as practicable with the entrance ofthe wetterl portion into the nip of the rolls so that the wetted surfacewill be in the nip of the rolls as the hydrophilic component of. thecoating swells.

(5) That the quantity of aqueous liquid applied be limited. so that theswelling of the hydrophilic material will take place beforesubstantiallyany of the aqueous liquid canpenetrate into the paper base.

(6) That the wetted surface of the coating remain in contact with the.polished drying surface until substantially dry.

Normally, when mineral-coated paper is densified by calendering thesurface produced looks reasonably level to the unaided eye. Under amicroscope, however, the surface is seen to be made up of juxtaposedhigh, glossy areas, and lower, less glossy areas.

Commonly, there are also visible under the microscope fairly numerousand relatively deep depressions or frothpits in the coated surface. Thesupercalendering operation. considerably decreases the thickness of thecoating originally applied and densifies it, accomplishing that resultby pushing nearer together the individual particles of mineral pigmentand thus decreasing the size, but not the number, of the intersticestherebetween. At the same timevthe elevation of the high spot above thelow spots is considerably decreased; froth-pits, however, are socomparatively large that little difference in their appearance isnoticed after the supercalenden'ng operation.

When such a supercalendered mineral-coated paper is passed through a nipwhich is kept filled or flooded with aqueous liquid, the quantity ofsaid aqueous liquid passing the nip apparently willbe the sum of thefollowing quantities; 1) the quantity which forms a liquid film betweenthe surface of the coating and the surface of the d'rum, (2) thequantity, which penetrates into the interstices between particles, and(3) the quantity which is actually absorbed by the hydrophilic materialpresent in the coating. Of, these three quantities the second and thirdare by far the most important in relation to the qualityv of thefinished product of the invention. In the case of properlysupercalendered mineral-coated paper the total quantity of aqueousliquid taken up in practicing the invention is so comparatively littlethat it can be readily retained at the surface of the coating or withoutpenetrating the underlying paper base. On the other hand, in the case ofa coating which has not been densified the roughness of the coatingpermits the trapping of a considerable quantity of aqueous liquidbetween the surface thereof and the surface against which it may bepassed in a nip, c.g. the finishing drum surfacea quantity which maybegreater thanthecoatinglayer, itself may be able to absorb completely.Moreover, when the interstices between pigment particles have not beenpartly closed, as by a calendering operation, they afford effectivepassages to the underlying paper base which are easily penetrable byaqueous liquid; and when the paper base once becomes wetted thereby, thewet fibers may then act as wicks to drain liquid from the coating. Thismay be the explanation of the noted fact that uncalenderedmineral-coated papers do not yield as satisfactory results when treatedby the polishing process of the invention.

Ideally in the practice of the invention the surface of the coatingacquires a mirror image of the polished surface against which it ispressed and dried. Variables which affect the degree to which the idealis approached are (l) the degree in which the hydrophilic adhesive orother hydrophilic material quickly swells, and (2) theexternal pressureapplied in the nip which forces the paper surface against the polisheddrying surface. Both variables aid. in obtaining intimate contactbetween the surface of densified coating and the surface of the drum.

The most feasible polished surface against which the mineral-coatedpaper may be pressed in the practice of the invention is a metal drum ofgood size, say 6 feet or more indiameter, which is internally heated,and suitably is plated with chromium, polished and maintained byconventional methods. For use to press the coated paper undergoingtreatment against the drum the most suitable means appears to be arubber-covered roll soft enough to ensure substantially even pressureacross the entire sheet or web of the paper. In such a set-up, a nip ofappreciable width, i.e., the distance from front to back, isproduced andwhile the nip pressure readily may amount to at least one hundred poundsper linear inch across the width of the sheet, such pressure is far lessthan the high pressures customarily used in supercalenders. If asupercalendered mineral-coated paper 0on taining no hydrophilic materialwhich is in the least swelled by aqueous liquid applied thereto, istreated according to theinvention, no amount of pressure expedient toapply will appreciably affect that sheet.

If, however, thehydrophilic matter present in the surface of the coatingis swelled, evento a very slight extent, by aqueous liquid appliedthereto, the coating is to some extent rendered moldable and whenpressed against the polished surface it is molded to conform to thesurface of the polished roll. In such a case the degree of deformationof the surface of the coating and consequent increase in polish or glossvaries with the pressure which is applied to it in the nip. Theglossobtainable by suchtreatment is very high indeed. The operationitself differs from conventional calendering, in that in part at least,the pressure which molds the surface of the sheet is supplied by theswelling of the hydrophilic component of the densified coating and thatthe swollen or moldable mineral-coating adheres firmly to the surface ofthe polished drum until the coating has become dry again. The effect ofthe treatment on the paper is that the glos imparted thereby is muchhigher than that obtainable by the severest supercalendering, butdinginess or blackening is not increased and may be decreased.

Since only a very small quantity of aqueous liquid passes through thenip, the drying problem is much less diflicult and relatively highspeeds can be attained even on a drum of small diameter and the expensefor equipment is reduced since polished drums even of smalldiameter costthousands of dollars. Moreover, and more important, because onlyv a verysmall amount of aqueous liquid is involved and has to escape as watervapor through the reverse side of the sheet, it is possible to turn thesheet over and subjectthe reverse side to the same process withoutappreciably affecting the finish previously developedon the first side.Thus the process of the invention is capable of producing mineral-coatedpaper having both ides h... WWQQQQUX enti al ppe an e n high gloss freefrom visible imperfections while by the prior processes it has beendiflicult or perhaps impossible to produce commercially a coated paperhaving satisfactory high gloss on both sides, except by flint-glazing orbufiing.

As the swelling increases, the surface is reshaped and molded againstthe polished drum. If the coating swells enough small froth-pits andother small surface defects originally present may even disappear. Thecoating after being dried may be found to have expanded slightly and tohave become slightly less dense; though the change is exceedingly small.The small decrease in density, however, is effective in decreasing orremoving dinginess or mottle that may have been present as a result ofsupercalendering. Concomitantly the brightness of the coating isimproved and the opacity may be slightly increased by the treatment. Thecoated surface embodying the present invention is free from thecross-grain microscopic scratches observable in the surface of glossyflint-gazed paper and from the longitudinal microscopic scratchesobservable on the surface of the glossy buffed paper made according toaforementioned Clark Patent No. 2,359,704. Moreover, the product doesnot show under a microscope the outlines of fibers which are oftenobservable in commercial so-called cast-coated papers, doubtless becauseunder the invention the paper base itself is not wetted by the aqueousliquid.

Because the coating of theproduct of the invention has been socompressed or densified, the product can be printed by regular printinginks commonly used for printing ordinary supercalendered, glossy,mineral-coated printing paper, and does not necessitate the use of thespecial transparent inks required for use on prior art cast-coated papernor such careful handling by the printer. The densified coating of thepaper embodying our invention does not absorb so much of the vehicle ofordinary printing ink into its interstices that the ink pigment left onthe surface is chalky and dull-looking. The fact that the product of thepresent invention, While retaining all the advantages of appearancepossessed by cast-coated paper, can at the same time be printed readilyand satisfactorily by use of regular printing inks, both letter-pressand lithographic inks, is a very valuable consideration from theviewpoint of the printer.

Another important advantage of the densified product of the invention isthat it is not much more subject to damage from breaking of the coatingduring handling than are conventional supercalendered mineral-coatedprinting papers. The papers embodying our invention do not have to behandled with special care while being manipulated in the press room.

The swelling of the hydrophilic coating material in making our coatedproduct may be controlled by variation of either one or both of twoseparate factors, namely; the composition of the coating itself and thecomposition of the aqueous liquid used. For example, variations can bemade in the particular kind and in the quantity of hydrophilic materialused in the coating. Commonly the hydrophilic material will be theadhesive, such as casein, glue, soy protein, polyvinyl alcohol, starch,etc., or mixtures of the same. The degree of swelling of the hydrophilicmaterial will likewise depend upon whether or not any water-proofing orinsolubilizing treatment has been given the coated product. Forinstance, it is common practice to subject the common hydrophilicadhesives used in coatings to the action of formaldehyde or otheraldehyde to render them more or less insoluble. Such treatmentdefinitely decreases the swellability of the material.

If the coating is found to be slow to swell upon application of wateralone, it is frequently found that an aqueous ammoniacal solution willbe much more eflicacious in swelling the material. On the other hand, ifan unusually susceptible material, such for example as a highly alkalinesodium caseinate, swells too much or takes up too much water, a littleformaldehyde dissolved in the aqueous liquid used to wet the coatingwill decrease the swelling to the desired degree.

As has been stated, water or aqueous liquid must not be drained awayinto the paper base before the hydrophilic matter has had time to swellagainst the polished surface of the drying drum. By proper selection ofthe aqueous liquid applied, and control of the quantity thereof as wellas by proper densification or calendering of the coating, it is possibleto practice the invention when the paper base itself has little or noresistance to being wet by water. It may be advantageous to have thepaper base surface-sized or surface filled, possibly because suchtreatment tends to lay down fibers which otherwise might project intothe coating layer and act as wicks to lead away water therefrom at alater step in the process.

In practicing this invention the maximum temperature to which the drumis heated becomes an important consideration. If the drum is heated to atemperature so that the aqueous liquid which is fed to the nip troughboils, a large part of the coating on thepaper entering the nip will beremoved and become incorporated in the aqueous fluid in the nip. If thissituation occurs the previously dried and compacted coating loses itsidentity and the resulting product is Worthless. The resultant surfacetends to resemble stippled plaster and has no finish which would becapable of receiving a printed impression.

It is desirable to maintain the temperature of the drum as high aspossible in order to facilitate the rate of drying of the small amountof aqueous liquid which passes through the nip. However, care must beexercised so that this liquid does not evaporate at a rate of sufiicientmagnitude to form steam bubbles between the surface of the coating andthe surface of the drum which would break the contact between these twosurfaces. The generally preferred drum temperature is from about toabout F.

It is believe that the most nearly perfect surfaces resulting frompractice of the invention occur in those cases in which the hydrophiliccoating swells the most quickly and to the greatest degree in thepressure nip. In such cases, however, it is desirable to avoidsubstantial swelling before the coating is pressed against the dryingsurface. This is accomplished by maintaining a pool of aqueous liquid inthe nip trough formed by the press roll and the polished roll. A pool inthe nip trough of an inch or two in depth is ample. The permissibledepth, however, depends in part upon the speed at which the paper passesthrough the nip; in most cases the speed will be at least 100 feet perminute. At this speed and at liquid depths of one or two inches thesurface of the coating is in contact with the aqueous liquid for onlyfrom $5 to $4 of a second before it passes into the nip and into contactwith the polished roll. By keeping short the period of contact betweenthe surface of the coating and the aqueous liquid the amount of materialremoved from the surface of the coating and washed away with the nipeffiuent is reduced. This holds the loss of coating weight to a mini-mumin this process.

Variations in glossy appearance of the product can be obtained byvarying the exact conditions of treatment, the optimum conditions ingeneral being best determined by experiment in each separate case.Nevertheless, very excellent appearance generally results in all caseseven when no attempt is made to reach absolutely optimum conditions. Thespecular gloss values of papers embodying the present invention may bemeasured by at Scott- Aminco goniophotorneter made by the AmericanInst-rument Company of Silver Spring, Maryland. 'In making the tests,the sample was illuminated by a beam of light making an angle of 70degrees from normal to the sample surface, and the reflected lightmeasured was at an angle also 70 degrees to the normal to the papersurface but on the opposite side of the normal, so that if theillumination was at plus 70 degrees the reflected light was at minus 70degrees to the normal. The reflected light was 7 passed through a lensand then through a slit aperture placed at the focal distance from thelens. The slit subtended an angle of 1 38' in the plane of the lightbeam and an angle of 5 2 across the plane of the beam, the angles beingfrom the optical center of the lens. The numerical figures so obtainedfor specular gloss are in percentages of the light reflected from ahighly polished flat black plate glass standard.

Measurements made as described upon usual supercalendered glossy-coatedprinting papers of the quality at present sold as No. 1 qualityglossy-coated printing papers usually yield values of specular glossbetween and 15. Especially severe supercalendering may yield papershaving specular gloss values of about 20. Never, so far as we know, dosupercalendered coated papers have a specular gloss as high as 25. Onthe other hand, the product of the present invention has a speculargloss, measured as described, of at least 30, and preferably of about 35or above. Most samples fall between 35 and 60 specular gloss, butfrequently specular gloss values over 60 are reached. It is found,however, that increasing values of specular gloss beyond a certain pointare not worthwhile because any sheet having a specular gloss as high as40 is so excellent in appearance that little further improvement eitherin appearance or in printing qualities is observable by the users.

The improvement in printing qualities may be determined quantitativelyby the tests for affinity for ink and "ink-setting time described inPatent No. 2,395,992 of John W. Clark. Briefly, it may be said that theinksett-ing time is time in seconds between the time the sheet isprinted with a standard printing-ink and the time the ink applied ceasesto look wet; it is a reliable indication of the receptivity of the sheetfor printing ink. Generally speaking, papers having an ink-setting timeof not over 40 seconds are considered to have excellent ink receptivity.The products of the invention usually show ink-setting times well under40 seconds. Glossy-coated papers which are finished by the flint-glazingprocess or by the bufiing process of the Clark Patent No. 2,439,704invariably have ink-setting times considerably in excess of 40 seconds,and are considerably more difficult to print satisfactorily than are theproducts of this invention. The product 'of the invention is also freefrom the characteristic scratches found in flint-glazed and buffedpaper.

Conventional mineral coatings can generally be treated according to theprocess of the invention provided they contain a hydrophilicwater-swellable component which is not too water-resistant. Conventionalpigment materials or mineral fillers suitable for use in coatings to betreated according to the invention include: clay, calcium carbonate,blanc, talc, titanium dioxide, ochre, iron oxide, ultramarine, coloredlakes and toners and the like. Some embodiments of the invention areshown by the following specific examples:

EXAMPLE 1 A coating composition was prepared of the followingingredients:

Parts by weight, dry

Fine paper coating clay 7 Fine paper coating calcium carbonate 30 Casein(solvated by ammonia) Water to make solids content 45%.

was removed from the drum when dry and was found to have a speculargloss value of 47.

EXAMPLE 2 A coating was prepared of the following ingredients:

Parts by weight, dry Fine particle English clay 30 Fine particle Georgiaclay 35 Fine particle calcium carbonate 35 Casein (solvated by Na P O 15Water to make solids content 44%.

This composition was applied to one side of a surfacesized paper bodystock of pounds weight per ream in quantity amounting to 16 pounds, dryweight, per ream and supercalendered to a normal supercalendered finish.The supercalendered surface was then run at feet per minute into a nipagainst a steam heated polished chromium-plated drum four feet indiameter while an aqueous solution of 0.4% formaldehyde was fed into thenip and maintained at a depth of about V2 inch. The drum surfaceadjacent the nip was about F. The paper was removed from the drum whendry and was found to have a specular gloss 'value of 50.4.

EXAMPLE 3 A coating composition was prepared of the followingingredients Parts by weight, dry

Fine coating clay 60 Fine calcium carbonate 40 Starch, enzyme converted16 Casein, solvated by ammonia 4 Styrene-butadiene copolymer (latexform) 4 Water to make solids content 46%.

This coating composition was applied to one side of a conventional paperbody stock of 55 pounds weight per ream in amount equal to 15 pounds perream, dry weight. The coated web was dried and supercalendered to a goodsupercalendered finish. The supercalendered surface was then run into anip against a heated chromium-plated drum while an aqueous solutioncontaining 0.6% of ammonia was fed into the nip. The paper was removedfrom the drum when dry and was found to have a high gloss and anattractive appearance. The specular gloss value was over 35.

EXAMPLE 4 A coating composition was prepared of the followingingredients Parts by weight, dry Fine paper coating clay 100 Casein(solvated by ammonia) 17.4 Ammonium stearate 0.65 Water to make solidsof 46.5%.

This coating was applied to one side of a surface filled paper of about51 pounds ream weight, about 15 pounds dry weight of coating beingapplied per ream. The paper was dried and supercalendered. Thesupercalendered surface was run into a nip with the coated side againsta heated chromium-plated drum while water was fed into the nip. Thesurface of the coating was dried in contact with the drum and releasedcleanly when dry. The specular gloss of the finished sheet was 59.5.

EXAMPLE 5 A coating composition was made of the following ingredients:

Parts by weight, dry

Fine paper-coating clay 65 Fine paper-coating calcium carbonate 35Casein (solvated by ammonia) 16 Water to make solids 42%.

This coating composition was applied to both sides of a surface-sizedpaper base of about 68 pounds ream weight, about 16 pounds, dry weight,of coating being applied to each side of the sheet. The paper was dried,and supercalendered on both sides. The supercalendered web was then runinto a nip with one side against a heated polished chromium-plated drumwhile the nip was kept flooded with an aqueous solution containing 1.0percent of dissolved ammonia and 0.25 percent of dissolved ammoniumstearate. The sheet was dried in contact with the polished drum and thenremoved. The sheet was then reversed and run into a nip with the othercoated side in contact with a heated polished chromium-plated drum whilethe nip was kept flooded with an aqueous solution containing 0.8 percentammonia, 0.4 percent hexamethylenetetramine and 0.25 percent of ammoniumstearate. The sheet was dried in contact with the drum and was thenremoved. The specular gloss of the first treated side was 40 and of thesecond treated side was 41.

Accordingly we believe ourselves to be the first to discover that asuperior high gloss can be produced on a supercalendered coated sheet bywetting the surface of the densified coating sufficiently to cause thehydrophilic component of the coating to swell and bringing the wettedsurface into contact with the highly polished surface of a heated rollunder pressure part of which may be subplied by the swelling of at leastpart of the hydrophilic component of the coating to swell and bringingthe wetted roll and a suitable backing roll, and that by so doing apaper sheet may be produced having on one or both sides a densifiedmineral-coating of a density comparable to a usual supercalenderedmineral-coating, but having a specular gloss of at least 30, andpreferably at least 35, as measured as described on a Scott-Amincogoniophotometer; and the coated surface being easily printable by thesame techniques and same printing inks customarily used in printingsupercalendered mineral-coated paper.

The invention is illustrated in the accompanying drawings in which:

FIG. 1 represents a greatly magnified cross-section of coated paperprior to supercalendering,

FIG. 2 represents a cross-section of the same paper aftersupercalendering, I I 1 FIG. 3 represents a cross-section of the samepaper after the gloss treatment, and

FIG. 4 is a flow sheet of the process. I

In FIG. 1, 1 is the paper body stock and 2 the coating comprising thepigment particles 3 and the adhesive 4. The porosity of the body stockand of the coating cannot be illustrated but as appears in FIG. 2 boththe body stock and the coating have been densified by reduction of thedegree of porosity. The number and size of the pigment particles has notbeen reduced but they have been brought into closer proximity to eachother. It will be seen further that the surface of the coating of thesupercalendered sheet in FIG. 2 is more nearly level than in FIG. 1. Asshown in FIG. 3 the coating has been slightly swelled or expanded by thegloss treatment and its surface has been further smoothed and levelled.

It will be appreciated that the illustrations in FIGS. 1 to 3 are notintended accurately to represent the structure of the paper but aregiven merely to show applicants conception of the elfects of thesupercalendering and gloss treatments upon the body stock and coating ofa sheet of coated paper.

In FIG. 4, 5 is a feed roller for the paper body stock, 6 is aconventional air knife coating unit, 7 a dryer, 8 a supercalender, 9 themolding drum having a highly polished chromium surface, 10 a pressureroller, 11 a windup roller and 12 the paper web. The movement of thepaper through the apparatus from the feed roller 5 to the windup roller11 is indicated by arrows. The pool of wetting liquid in the nip betweenthe molding drum and the paper web is shown at 13.

It is believed that in carrying out the preferred method of thisinvention a thin film or layer of aqueous liquid is formed on thesurface of drum 9 as the coated paper passes through the nip of thepress roll surface 10 and 10 the drum surface. Hence it is believed thatthe wetted surface of the coating after passing through the nip pool ispressed against a thin film or layer of aqueous liquid on the drumsurface and as the paper revolves with the hot drum surface, water fromboth said aqueous liquid in the film and the wetted coating surface isevaporated and passes through the paper as vapor. The terms staticadhesive contact with, static contactwith, immovable contact with andforcible contact with a polished surface, polished finishing surface orthe surface of the finishing roll as used in the specification andclaims include contact with such a surface which carries a thin film orlayer of the aqueous liquid which is supplied to the nip of the pressand the finishing rolls.

This application is filed as a continuation in part of our applicationSerial No. 210,214, filed February 9, 1951, which was a substitute forour application Serial No. 207,112 filed January 22, 1951, both nowabandoned.

Although it has been attempted to explain the theory of the presentinvention, it is not intended that the present invention be limited tosuch theory.

It should be understood that the present disclosure is for the purposeof illustration only and that the present invention includes allmodifications and equivalents which fall within the scope of theappended claims.

We claim:

[1. A method comprising applying aqueous liquid to the dry surface ofthe coating of a supercalendered coated paper, the coating of whichcomprises a water swellable hydrophilic material, subsequently pressingthe resulting moist surface into static adhesive contact with a highlypolished finishing surface and drying the paper while in contact withsaid finishing surface] [2. A method comprising applying aqueous liquidto the surface of the coating of a dry, supercalendered coated paper,the coating of which contains a water-swellable hydrophilic material, asnearly simultaneously as possible with the application of said liquid,pressing said surface into static contact vw'th a highly polishedfinishing surface and drying the paper while in contact with saidfinishing surface] [3. The process of claim 2 wherein said finishingsurface is wet with aqueous liquid when said surface of the coating ispressed thereagainst] [4. A method comprising applying a coatingcomposition comprising a water swellable hydrophilic material to a webof paper, drying the resulting coated paper, supercalendering the driedcoated paper, passing the surface of the coating of said dried,supercalendered coated paper through a pool of aqueous liquid, directlythereafter without removing the same from .said pool pressing saidsurface into static adhesive contact with a heated, polished finishingsurface and drying the paper while in contact with said finishingsurface] [5. The process of claim 4 wherein said finishing surface ismaintained wet with aqueous liquid when said surface of the coating ispressed thereagainst by passage through said pooL] [6. In a processcomprising applying aqueous liquid to the dry surface of the coating ofa coated paper, the coating of which comprises a swellable hydrophilicmaterial, subsequently passing said coated paper through a nip between ahighly polished roll and a backing roll with the coating facing saidpolished roll and drying the paper while in static adhesive contact withsaid polished roll, the improvement comprising drying and subsequentlysupercalendering said coated paper before applying aqueous liquid to thesurface of the coating and applying said aqueous liquid to said surfaceof the coating as it passes into said nip.]

[7 The process of claim 6 wherein said aqueous liquid is applied to saidsurface of the coating as it passes into said nip by maintaining a poolof aqueous liquid in said nip] [8. In a process comprising applyingaqueous liquid to the surface of the coating of a coated paper, thecoating of which comprises a swellable hydrophilic material andsubsequently passing said coated paper through a nip between a highlypolished roll and a backing roll with the coating facing said polishingroll and drying the paper while in static adhesive contact with saidpolished roll, the improvement comprising drying and subsequentlysupercalendering said coated paper before applying aqueous liquid to thesame] [9. That improvement in the art of making polished paper whichcomprises compressing dry coated paper by super-calendering, passing thecompressed paper between a press roll and a highly polished finishingroll, wetting the surface of the coating with aqueous liquid as thepaper comes into contact with the finishing roll and drying the wettedsurface of the coating while it is in im movable contact with thepolished surface of the finishing roll, said coating comprising a waterswellable hydrophilic material] [10. That improvement in the art ofmaking polished paper which comprises supercalendering dry coated paper,swelling a hydrophilic component of the coating by the application ofaqueous liquid to the surface of the coating, and while the swelling istaking place and before the body stock has been swollen bringing thewetted surface of the coating into forcible contact with a polishedsurface and drying the moistened surface while still in static adhesivecontact with the polished surface] [11. That step in the process ofmaking highly polished mineral-coated paper which consists in swellingthe surface of a hard dry supercalendered sheet by the application ofmoisture to the coating as nearly as possible at the moment when thesheet passes between a pair of rolls one of which has a highly polishedsurface and drying it while in static adhesive contact therewith, saidcoating comprising a water swellable hydrophilic material] [12. A methodcomprising applying aqueous liquid to the dry surface of the coating ofcoated paper which has been calendered with enough pressure to densifythe coating and to render the surface fairly level, said coatingcomprising a water swellable hydrophilic material, subsequently pressingthe resulting moist surface into static adhesive contact with a highlypolished finishing surface and drying the paper while in contact withsaid finishing surface] [13. A method as defined in claim 12 in whichthe aqueous liquid is applied to the coating by passing the paperthrough a pool thereof which is in contact with said finishing surface][14. A method as defined in claim 12 in which the aqueous liquid isapplied to the coating by passing the paper through a pool thereof whichis supported in the nip betweeen a highly polished roll and a backingroll] [15. A high gloss mineral-coated paper substantially identicalwith the product obtainable by the method defined in claim 12.]

16. The method of forming a high specular gloss on the surface of acoating on a paper web, the coating comprising a warer-swellablehydrophilic adhesive material and having a smooth, dry, densifiedsurface, which method includes the steps of: moving the paper web towardand pressing said coated surface against a polished heated finishingsurface, applying a limited amount of an aqueous liquid to the surfaceof said coating during movement of the web so that liquid will be pickedup by said coating to swell only the coating surface; limiting thequantity of said liquid picked up by the surface of said coating to anamount no greater than that which can be readily absorbed by saidcoating to prevent penetration of said liquid through said coating andinto the paper to an extent which would cause wetting and swelling ofthe paper web; pressing the surface of said coating while at least thesurface portion of said coating is swelling and while at least a portionof said liquid is at the surface of said coating, against said polishedheated finishing surface; and drying said coating while the surface ofthe same is in adhesive contact with said polished heated finishingsurface.

17. The method of forming a high specular gloss on the surface of acoating on a paper web, the coating comprising a warer-swellablehydrophilic adhesive material and having a smooth, dry, densifiedsurface, which method includes the steps of: applying to said coatingsurface a limited amount of an aqueous liquid capable of penetratinginto and swelling only said coating surface; pressing said coatingsurface against a polished heated finishing surface while said liquid ispresent at, penetrating into and swelling said surface; limiting thetotal quantity of said liquid penetrating into said coating before,during and after said pressing step to an amount which can be readilyabsorbed by said coating without wetting and swelling said backingsheet; and drying said coating while the surface thereof is in adhesivecontact with said polished heated finishing surface.

18. The method of forming a high specular gloss on the surface of acoating on a paper web, the coating comprising a water-swellablehydrophilic adhesive material and having a smooth, dry, densifiedsurface, which method includes the steps of: moving the coated surfaceof said paper web toward and against a polished heated finishingsurface, applying a limited amount of an aqueous liquid to the saidsurface of said coating during said movement against said polishedsurface so that when said coating is pressed against said polishedsurface said liquid will be carried forward with said coating as aliquid film. between said coating and said finishing surface, as freeliquid in interstices between particles in said coating and as liquidabsorbed only into said hydrophilic material; limiting the quantity ofsaid liquid so carried forward with said coating to an amount no greaterthan that which can be readily absorbed by said coating to preventpenetration of said liquid through said coating and into the paper to anextent which would cause wetting and swelling of the paper web; pressingthe surface of said coating while a portion of said liquid and at leastthe surface portion of said coating are swelling, against said polishedfheated finishing surface; and drying said coating while the surface fof the same is in adhesive c ntact with said polished heated finishingsurface.

19. A high gloss mineral coated paper substantially identical with theproduct obtainable by the method defined in claim. 18.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS

